1. Field of the Invention
This invention relates to Joule effect heating of thermoplastic material wherein two or more power sources are each connected to at least two electrodes engaging the thermoplastic and more particularly to the control of the heating derived from current flow within the material and between the sources.
In the discussion of this invention and its various aspects, glass will be employed as the exemplary thermoplastic material upon which Joule effect heating is applied. However, it is to be appreciated that the method and apparatus of this invention lends itself to other thermoplastic materials and therefore the invention should be considered to be generally applicable to such materials.
2. Description of the Prior Art
In the manufacturing of glass, an electric furnace may be utilized to melt a batch of raw materials in a refractory lined furnace chamber. Although hydrocarbon fuel burning furnaces may also be utilized to produce glass, the electric furnace has certain advantages with respect to the problems of air pollution and maintenance of uniform heating.
Typically, an electric furnace will have two or more electrodes submerged in the molten glass which are connected to a source of alternating current. The resistivity of the molten glass transfers the electrical energy of the current flowing between electrodes into heat energy thereby creating Joule effect heating. Molten glass has a negative temperature coefficient and therefore, the resistivity below a critical temperature is sufficiently high so as to limit current flow below a level at which electric melting can be sustained. The power supplied to the furnace chamber is regulated as by time proportioning or by phase controlling the applied voltage with suitable means, typically silicon controlled rectifiers. Since, during normal operation, the control is operating at 92% to 95% of maximum for efficiency of equipment utilization and, in the case of phase control, is conducting over that range of the voltage cycle to obtain a favorable power factor, neither the current flow nor the voltage can be increased significantly to raise the temperature of the molten glass. Thus, for example, when the molten glass falls below the critical temperature, its molten state has not been maintained electrically. Therefore, electric furnaces generally require a plurality of fuel burners positioned to direct radiant heat to the upper surface of the material in the furnace chamber. This radiant heat melts the material until the critical temperature is reached above which the resistivity of the molten glass is low enough to permit sufficient current to flow between the electrodes for normal controlled electric heating furnace operation.
Conditions occur in electric furnace operation where the temperature of the molten glass falls below that critical temperature of the system from which Joule effect heating derived from the sources of electrical power supplying the electrodes is sufficient to raise the temperature. At some given temperature for a given glass the resistance of the glass between electrodes connected to the same source becomes so high that the available voltage is insufficient to provide Joule effect heating at a rate exceeding the heat loss. In the aforenoted related patent applications there is disclosed a method of an apparatus for applying increased voltage across restricted regions of the glass mass by interconnecting electrodes supplied by separate sources to effectively couple those sources in series aiding relationship across glass between electrodes of different mated pairs. In the case of like phased sources of alternating current the interconnection of electrodes of different mated electrode pairs which are of opposite polarity by a low impedance path such as a cable effectively doubles the voltage imposed across the glass mass between the other electrodes of the different mated electrode pairs. The interconnection of differently phased sources can also be effective to impose increased voltage across the glass mass of electrodes connected to the different sources by a series aiding relationship. Thus where sources are shifted in phase 60.degree. and electrodes are interconnected so that their instantaneous voltage values are spaced 120.degree. in phase the effective voltage will be 1.732 times the individual source voltages, where like source voltage magnitudes are involved. The same sources will impose a voltage equal to the source voltages when the electrodes which are connected have their instantaneous voltage values spaced 60.degree. in phase.
The increased applied voltage imposed by the interconnection, while effective to locally increase the power dissipation in the glass between the electrodes, can produce a runaway condition if it is retained as the Joule effect heating is effective to raise the glass temperature. The negative temperature coefficient of the glass will result in a reduction of the resistance in the circuit to which the added voltages are applied to a degree which can exceed the capacity of elements of the electrical power sources, for example beyond the capacity of transformers or silicon controlled rectifiers. Runaway conditions can be avoided by careful monitoring of the electrical parameters or the thermal conditions in the portion of the system affected and by disconnection of the interconnection between sources, as by opening a switch in the cable when the desired result has been realized. The runaway conditions can also be avoided by reducing the source voltages through their individual controllers, however, this also reduces power to the primary heating zones between mated electrodes.
The systems disclosed in the related applications provide various intergroup voltages between electrodes connected to different sources depending upon phase relationships so that some degree of control of the intergroup voltage magnitudes imposed on the glass is afforded by the phasing between electrodes which are interconnected. In one example of three pairs of electrodes in a tank for heating molten glass supplied by delta connected transformer primaries from a three phase source so that the individual supplies are phased 120.degree. apart, the voltages applied to the electrodes contacting the glass can be adjusted in their relationships as by the polarization of the transformer secondaries and the connections to the electrodes so that electrodes of adjacent pairs have voltages which are shifted 60.degree. or 120.degree. in phase with respect to each other. That is a connection can be made between electrodes of each pair of electrodes which have instantaneous voltages shifted 120.degree. in phase. In such an arrangement, the voltage between the other two electrodes is increased to 1.732 times the individual source voltages applied, or by an alternative connection of electrodes having voltages shifted 60.degree. in phase the voltage across the other electrodes will be increased to the value of the individual source voltages applied. These adjustments can be made selectively to provide some degree of interzone applied voltage control. However, they are limited to fixed voltage steps for the interphase voltages derived from any given combination of source voltages connected to the mated electrodes.
An object of this invention is to facilitate the electric heating of thermoplastic materials, particularly materials with a high temperature coefficient of resistance such as the negative coefficient of molten glass.
A second object is to enhance the control of electric currents within molten thermoplastic materials which are derived from a plurality of separate current sources.
A third object is to expand the range of control of interphase electric currents derived from a plurality of polyphase sources and imposed on molten thermoplastic material.
A fourth object is to afford flexibility in shifting from one magnitude of glass throughput to another without loss of thermal control by Joule effect heating.
A fifth object is to effectively utilize the power supply at or near its rated capacity throughout a wider range of operating conditions than heretofore possible.